1. Field of the Invention
This invention relates to an image signal processing apparatus for processing image signals.
2. Description of the Prior Art
Heretofore, as an apparatus for processing image signals there has been known a digital image-signal processing apparatus which digitizes image signals, once stores the digitized signals in a field memory, and outputs the contents stored in the memory by performing proper processing. The apparatus may perform special reproductions, such as still reproduction in a video tape recorder and the like, so that noise is not generated on a display picture surface, and can be utilized in special effects, such as multiple-picture-surface displayed in which plural images are simultaneously displayed on one display picture surface, wipes, fades and the like, or can be utilized in a noise reducer, which performs signal operation over several fields to reduce a noise component, and the like. FIG. 1 shows a block diagram of a cyclic-type noise reducer as a conventional image signal processing apparatus. In FIG. 1, an image signals generated from a camera unit 2 is, for example, converted from an analog signal into a digital signal by an analog-to-digital (A/D) converter 3. The digital image signal is multiplied by a coefficient (1-K) at a coefficient multiplier 12 within a processing circuit 14, and is supplied to a field memory 6, which will be described later, via an adder 13. The signal read from the field memory 6 is multiplied by a coefficient K at a coefficient multiplier 11, added at the adder 13 to the signal which has been multiplied by (1-K) at the coefficient multiplier 12, and the resultant signal is output, the circuitry thus constituting a cyclic loop.
A digital-to-analog (D/A) converter 10 converts the digital image signal output from the adder 13 into an analog image signal and outputs the analog signal at 8. A synchronizing-signal separation circuit 4 separates a synchronizing signal added to the image signal. A memory controller 5 controls conversion operations in the A/D converter 3 and the D/A converter 10 in accordance with the synchronizing signal separated at the synchronizing-signal separation circuit 4, and assigns a write address or a read address in the field memory 6 for the digital image signal output from the adder 13. The field memory 6 stores digital image signals in units of a field. An operation unit 9 assigns various kinds of modes, such as noise reducer processing, wipe processing, fade processing and the like. A coefficient generator 7 supplies the coefficient multipliers 11 and 12 with the coefficient K complying with the mode assigned by the operation unit 9, and sets the values of the coefficients 1-K and K.
In FIG. 1, an image signal including a noise component which is generated from the camera unit 2 is digitized at the A/D converter 3, and then multiplied by (1-K) at the multiplier 12, and stored in the field memory 6 via the adder 13. The signal read from the field memory 6 is multiplied by K at the multiplier 11, supplied to the adder 13, and added to the signal multiplied by (1-K) at the coefficient multiplier 12. Thus, only a random noise component which is randomly generated for every field period in an image signal is suppressed.
Now, in the above-described circuit configuration, when performing wipe or fade operation of a still-picture image and a moving-picture image, it is necessary to store image signals obtained from the camera unit 2 in the field memory 6. When writing image signals output from the camera unit 2 in the field memory 6, input is performed via the processing circuit 14. Hence, after initial setting of the value of the coefficient K at the coefficient multipliers 11 and 12 has once been performed and storage in the field memory 6 has been completed, the value of the coefficient K to be used by the coefficient multipliers 11 and 12 is set by the coefficient generator 7 in accordance with the mode assigned by the operation unit 9, and a period for setting the value of the coefficient K is therefore required. Accordingly, storage operation of image signals obtained by the camera unit 2 in the field memory 6 is delayed by the time needed for performing the initial setting of the value of the coefficient K, and the image signals can not be instantaneously stored in the field memory.